In this study, the adsorption property of magnetic graphene oxide grafted 3.0 generations polymaleicamide dendrimer (GO/Fe3O4-g-G3.0) nanohybrids for Hg(II) in aqueous solution was investigated by batch experiments and magnetic separation technology. The results showed that Hg(II) ions were partially reduced to Hg(I) in the adsorption process, the solution pH value had a major impact on Hg(II) adsorption, and the higher temperature was beneficial to the adsorption of GO/Fe3O4-g-G3.0. The adsorption kinetic was well-fitted by the pseudo-second-order kinetic equation and the Langmuir adsorption isotherm model correlated well with adsorption equilibrium data.